Conventionally, there has been a radio-frequency identification (RFID) tag system which obtains ID information from a RFID tag that passes through plural gates disposed close to each other. The RFID tag system includes a RFID reader which has transmission means, obtaining means and measurement means. The RFID reader has one or plural of antennas that are provided at the gate. The transmission means transmit a read command to the RFID tag when the RFID tag enters the gate. The RFID tag outputs a response signal when the RFID tag receives the read signal. The obtaining means receive the response signal from the RFID tag and obtain the ID information of the RFID tag. The measurement means measure power of the response signal.
However, there is a problem in that the RFID tag system may receive the ID information from the RFID tag which does not pass through the gate when the reader receives the ID information of other RFID tags passing through the gate. Such a problem may occur in a case where the RFID tag is provided close to the gate, for example. In this case, the RFID tag which is provided close to the gate and does not pass through the gate is an unwanted tag. In order to solve the problem, there is a RFID tag system which includes a determining part that determines whether a detected RFID tag is an unwanted tag based on a time series change of power received from the detected RFID tag, and a filtering part that filters out the ID information of the unwanted tag.
The conventional RFID tag system has a threshold value of signal intensity, and recognizes the detected tag as an intended tag which is to be read in a case where the signal intensity obtained from the detected tag when the detected tag passes through the gate exceeds the threshold value and where a time series change of the signal intensity includes only one peak.
However, many kinds of patterns of the time series changes of the signal intensity may be obtained when the detected tag passes through the gate in actual operation, and it is not always true that only one peak is obtained in the time series changes of the signal intensity. Accordingly, in the conventional RFID tag system, it is difficult to determine whether the detected tag is the intended tag or the unwanted tag.
In a case where the conventional RFID tag system is installed at a gate through which an article with the tag attached passes, the unwanted tag is not a type of tag which passes through the gate and is read by the system at the gate, but is a type of tag which does not pass through the gate but is read by the system near the gate.
In a case where it is difficult to determine whether the detected tag is the intended tag or the unwanted tag, reading accuracy of the system will be degraded.